This application claims priority under 35 U.S.C. xc2xa7119 on German patent application 10125397.4 filed May 23, 2001, the entire contents of which are hereby incorporated herein by reference.
The invention generally relates to a method for drilling micro-holes. More preferably, it relates to drilling micro-holes in a multi-layer substrate having a first metal layer and at least one second metal layer, and having a dielectric layer in each case arranged between two metal layers. The drilling may be done, by irradiation with the energy beam of a solid-state laser, and may be carried out in two operations. In the first operation, the first metal layer and a part of the underlying dielectric layer may be in each case, ablated and, in the second operation, the dielectric layer is ablated cleanly down to the second metal layer.
As electrical circuit substrates continue to be miniaturized, it is increasingly necessary to make both through-holes and blind holes having diameters of less than 200 xcexcm, which can scarcely still be made with mechanical drills or punch needles. In this field, it has already been customary for a long time to produce micro-holes using laser drilling. However, this entails the problem that the different materials, i.e. conductor materials such as copper, on the one hand, and dielectrics, for example polymers with and without glass-fiber reinforcement, on the other hand, place very different demands on the laser processing.
For instance, it is well known which lasers in which wavelength range are optimally suitable for drilling in metals or for drilling in plastics. Problems arise, however, when multi-layer substrates need to be drilled through with one and the same laser, or need to be provided with blind holes to produce a conductive connection with a metallic interlayer. When such different material layers are being drilled, negative thermal effects can occur, for example detachment effects between metal layers and dielectric layers, damage to the dielectric itself or inadvertent drilling through a metal layer with which contact is intended to be made using a blind hole.
U.S. Pat. No. 5,593,606 A describes a method for drilling micro-holes in a multi-layer substrate, with which a UV laser with one and the same dimensioning is to be used for drilling completely through at least two layers having different properties. If the laser beam is dimensioned in such a way that it drills through a first metal layer and subsequently completely ablates a dielectric layer, however, this directly entails the risk that it will not come to a stop at the correct time on the second metal layer. Therefore, the second metal layer will also be attacked to a greater or lesser extent, unless the fact that the desired drilling depth has been reached is established by appropriate measuring instruments and the drilling process is terminated at the correct time.
In order to counteract this problem, U.S. Pat. No. 5,841,099 proposes a two-stage method (for drilling two layers), the laser being set to a higher energy density in a first operation for drilling a first metal layer, and the energy density of the laser being reduced, in the second operation for drilling a dielectric layer, to the extent that it falls below the threshold for metal vaporization. Because of its lower energy density, the laser can no longer drill through a second metal layer adjoining the dielectric layer. In order to set this lower energy density in the second operation, it is proposed therein to increase the repetition rate of the laser. However, optimum utilization of the laser energy and an optimum processing rate are not obtained in this way.
It is therefore an object of an embodiment of the present invention to refine the two-stage method, mentioned in the introduction, for drilling micro-holes using a laser. Preferably, this is done such that the micro-holes can be made with good quality and the highest possible processing rate, while optimally utilizing the laser power.
According to an embodiment of the invention, this object can be achieved by a method of drilling micro-holes in a multi-layer substrate having a first metal layer and at least one second metal layer, and having a dielectric layer in each case arranged between two metal layers. Drilling can be done by irradiation with the energy beam of a solid-state laser having a repetition frequency of at least 10 kHz, a wavelength of less than 1100 nm and a pulse length of less than 50 ns. The irradiation can be carried out in two operations in such a way that, in the first operation, the first metal layer and a part of the underlying dielectric layer are in each case ablated and, in the second operation, the dielectric layer is ablated cleanly down to the second metal layer.
In the first operation, the laser beam can be set to a repetition frequency of at least 15 kHz, focused onto the first metal layer and moved with a first circumferential velocity in a circle corresponding to the diameter of the desired hole. Movement can occur in such a number of passes until at least the first metal layer is cut through, the metal layer being fully removed in the hole region.
In the second operation, the laser beam can be set to an equal or lower repetition rate than in the first operation, directed out of focus onto the dielectric layer exposed in the hole and moved, with a circumferential velocity which is higher than the first, in one or more concentric circles inside the desired hole diameter. Movement can occur in such a number of passes until the dielectric layer is ablated in the hole region. The defocusing and/or the second velocity can be set in such a way that the effective energy density in the second operation lies below the threshold for ablation of the second metal layer.
In the method according to an embodiment of the invention, the repetition frequency is hence not increased, as in the prior art, to reduce the effective energy density when ablating the dielectric in the second operation. Rather, it is preferably lowered or at most kept at the same value as in the first operation. The effective energy density is instead reduced by defocusing, and hence by increasing the spot diameter on which the laser beam is incident, and furthermore by increasing the circumferential velocity, which shortens the action time of the individual laser pulses on a given area.
For drilling the hole in the metal layer (copper layer) in the first operation, it is generally sufficient for the laser to be moved repeatedly in a single circle, corresponding to the diameter of the desired hole, until the metal layer is circularly cut. For diameters up to 150 xcexcm, the metal core that has been cut out can then detach automatically because of the heating effect, and pop out. For larger hole diameters, an additional pulse for heating may be delivered onto the metal core that has been cut free.
In order to achieve a clean hole edge, a high overlap ( greater than 50%) of the individual pulses that form the circle is sought in the first laser drilling operation. To that end, a higher repetition frequency of at least 15 kHz, preferably between 20 and 30 kHz, is selected for this first operation. Admittedly, available lasers no longer deliver the maximum average power in this range. Nevertheless, a neodymium vanadate laser (Nd:VO4 laser) is preferably used, for which the power drop at higher repetition frequencies is still relatively minor. For instance, with a 355 nm Nd vanadate laser of 3.5 W having a focal spot diameter of 12 xcexcm, it is possible to achieve a linear circumferential velocity of  greater than 175 mm/s. With lasers having a higher power, even higher velocities can be achieved by virtue of higher repetition frequencies. It is also correspondingly advantageous to use a neodymium vanadate laser having a wavelength of 532 nm.
In the second operation, the dielectric material may be ablated by guiding the laser in at least two concentric circles, in which case no overlap of the successive laser pulses is necessary. Here, the repetition frequency is selected approximately in such a way that the maximum available laser power is utilized for the material ablation. This maximum power is obtained, as is known, at a slightly lower repetition frequency, i.e. at about 10 to 20 kHz in the case of Nd:VO4 lasers. In this case, the effective energy density can be matched, as mentioned, to a value below the vaporization threshold for metal by increasing the irradiated spot diameter, i.e. by defocusing or by altering the magnification factor of the collimator. In addition, the circumferential velocity is preferably increased in such a way that the individual pulses are no longer incident just on a spot corresponding to the beam diameter, but rather distribute their energy over a larger area owing to a blurring effect.
When using an Nd vanadate laser having a higher power, it is also conceivable not to make the laser beam move around a circle in the second operation, but rather to broaden the beam to the extent that it covers the entire hole area. In this case, the dielectric can be ablated with a central setting of the beam, with the hole cut out from the metal layer in the first operation being used as a mask.